Piezoelectric Transducers are used in a variety of applications as sensors in physical systems for initiating some type of system activity. In some applications mechanical forces are employed to mechanically strain or deform the piezoelectric element. The piezoelectric element converts the strain to an electrical signal which is a function of the applied force. In other applications an electrical signal which is coupled to the piezoelectric transducer results in a mechanical deformation of the piezoelectric element which is some function of the magnitude of the electrical signal applied thereto.
Piezoelectric transducers are employed as the mechanical force sensor in pressure sensing systems, strain gages, microphones and burglar alarms, for example, and as the electrical actuator in buzzers, speakers, ultrasonic heads, print heads, etc., to name only a few of the many applications. The Piezoelectric effect is known to be the phenomenon of certain materials which convert mechanical stress therein to electricity or which convert an electrical signal coupled thereto to a mechanical strain in the piezoelectric element itself.
An additional application of the piezoelectric effect is in the sensing of acceleration. Here, again, a force is coupled to the piezoelectric element as a function of the acceleration. Accelerometers are made by coupling a mass to a piezoelectric element such that the mass stresses, i.e., strains and deforms the piezoelectrical element when subjected to acceleration. The resulting voltage or electrical charge appearing across the piezoelectrical element is carefully amplified and can be used as an acceleration proportional voltage. The use of piezoelectric materials in sensing of acceleration is well known and understood.
It is desirable in many applications to test the piezoelectric element to be certain that it is functional before installation in a physical system and also to be able to test the element from time to time after installation in the physical system. Examples of such physical systems are seen in FIGS. 1, 2 and 3 which represent typical applications of piezoelectric transducers.
FIG. 1, in diagrammatic form, fragmentarily illustrates a sheet steel rolling mill involving the two final stages of the mill, represented in the roll stands RS1, RS2, and a take-up reel TR. The roll stands and the take-up reel are driven by respective motors M1, M2 and M3. The steel sheet exiting a roll stand is moving faster than that portion entering the roll stand, so successive stages of roll stands must be precisely speed regulated to maintain proper sheet tension to avoid sheet buckling or sheet breaking. For this reason a transducer 7 is employed, in what is sometimes called a tensionometer control system, to respond to the variable applied force F.sub.a as a result of sheet tension. As seen in FIG. 1, the transducers 7 have their outputs coupled to the respective control system sections which respectively control the windings W for the respective motors. While not shown, provision is made to simultaneously speed regulate the motors so that they operate close to the required speed. The fine speed control is provided by the piezoelectric transducer as a function of strip tension. In this application of the transducer, the piezoelectric element is directly strained or deformed by the tension in the sheet, resulting in the force F.sub.a coupled thereto.
As further background of this invention, FIG. 2 illustrates the application of a piezoelectric element in an accelerometer in a disk drive. The disk drive illustrated in FIG. 2 is a rotary actuator type of disk drive comprising a disk 3 and a rotary actuator 5 which mounts a transducer such as a magnetic head 5d. An actuator motor M4 having a winding W powers the actuator arm 5 to move the magnetic head 5d to different radial locations across the surface of the disk 3. Control of the excitation of the winding W to achieve such magnetic head positioning is under the control of a control system 10 which responds to requests from a host computer 15 by translating requests for data or, for writing data to tracks and track sectors on the disk. Portable applications of disk drives in personal computers, such as the popular "palm top" and "lap top" personal computers, present an abusive environment for the disk drive, where bumping and even dropping of the personal computer during operation can result in shock forces displacing the magnetic head from the track it may be following in retrieving data or writing data. In a rotary actuator type of drive the transducer is in the form of a rotary accelerometer 7 to respond to angular acceleration resulting from such shock coupling. The output of rotary accelerometer 7 is coupled to the control system to excite the winding W in a sense and in a degree which produces a torque which opposes and ideally balances the component of torque due to shock acting about the pivot of the rotary actuator 5.
A further application of a piezoelectric accelerometer, in this case a linear accelerometer, is found in an air bag deployment system application in a motor vehicle. Such an application is seen in FIG. 3. Here, the accelerometer 7 is coupled as input to a control system 10, forming part of the air bag deployment system, having an output which controls a valve 12b in an air bag inflater 12. The air bag inflater comprises an air supply 12a which is coupled by the valve 12b to an air bag 12c. In these installations, the threshold of response of the accelerometer is usually set at about 50 gs. Thus, in a crash situation exceeding the accelerometer threshold, the accelerometer 7 provides the signal which is the stimulus for operating the valve 12b, to quickly inflate the air bag.
In all such applications, it is desirable to know that the piezoelectric element is functional for its intended purpose, from the point of view of its functionality prior to installation in the system and as to its functionality after installation and use, or intended use, in the system. This is particularly important in the vehicle air bag system where testing of the accelerometer without deploying the bag is a desirable precaution.